The present invention relates to a method for detecting an AC/DC current without electrical contact by using the magnetic coupling in an iron core. The present invention may be used in zero-phase current detectors, or various kinds of leakage current detectors.
A zero-phase current transformer using earth-leakage circuit breakers, and earth-leakage protective relays detects small current without electrical contact. FIG. 19 shows the basic configuration of a prior art zero-phase transformer. As shown in FIG. 19, the zero-phase transformer comprises two conductors 2 passing through the center hole of an annular iron core 1 which has a high permeability. A resistor 4 is connected between the ends of detection coil 3 which is wound on iron core 1. Each of the conductors 2 is connected between AC source 5 and load 6 of the main circuit. Under normal conditions, forward and backward currents, I.sub.1 and I.sub.2, respectively flow in the two conductors 2 and have the same value, but are opposite in direction, as shown by the arrows in FIG. 19. Thus, iron core 1 is not magnetized and no voltage is induced in detection coil 3. With earth leakage in the load 6 side, the values of the currents I.sub.1 and I.sub.2 become different, magnetizing iron core 1 and inducing a voltage. As a result, a current flows in resistor 4 and the resulting voltage across resistor 4 can be taken out as a control signal. Although FIG. 19 shows a single phase, it is understood that the same theory can be applied to three phases if three sets of conductors 2 are provided. The current which can be detected by the zero-phase transformer is, theoretically, only an AC zero-phase current. The zero phase transformer cannot be used for a DC current.
The following prior art methods have been proposed to detect a DC current without electrical contact with the main circuit. Apparatus to realize these methods is not shown.
One of the methods is a DC current transformer which uses two closed magnetic circuit iron cores. The iron cores are preliminarily excited by an AC current to have magnetic flux directions opposite to each other. Where a DC magnetic field caused by a detected current is applied, the AC current changes. As a result, the DC current can be detected based on the change in the AC current.
Another method is a Hall element detector. According to the Hall element method, a current to be detected can be measured by a Hall element inserted in a gap in an iron core.
Recently, preventive maintenance has been regarded as important, so there has been a demand for earth-leakage circuit breakers, earth-leakage protective relays, and like ground fault circuit breakers, for DC appliances. However, the aforementioned DC current detecting methods cannot respond to this demand.
The current which can be detected by the zero-phase transformer is only an AC current as described above. Further, where such an AC current is detected, the detected current is small. The detected magnetic field is then also small, as shown in the magnetization curve in FIG. 20, even though a high-premeability material such as Fe-Ni alloy, for example, Permalloy (tradename), is used as iron core 1. Accordingly, the magnetic flux obtained is low, so that the size and weight of the detector must be increased.
The method of detecting a DC current using a DC current transformer is inappropriate because two iron cores are required. This increases the size of the detector. Therefore, this method is theoretically unsuitable for a small current even though it may be suitable for a large current.
The method of detecting a DC current using a Hall element detector is inappropriate because the gap in the iron core is easily influenced by an outside magnetic field. Therefore, magnetic shielding is required. Accordingly, the size of the detector is increased. Further, when the main current is large and the current to be detected is small, as in a zero-phase transformer, the magnetic field induced by the main current cannot be balanced to zero because of the influence caused by the gap in the iron core.
As described above, no apparatus suitable for detecting a DC differential current has been provided. In addition, the DC current to be detected for controlling earth-leakage circuit breakers, earth-leakage protective relays, and the like, may not be a perfect DC current but may be a pulsating current obtained by rectifying an AC current.